Environmental Problems, Their Causes, and Sustainability 1

peated scraping. Slow-growing, long-lived corals,sponges, and fish are particularly vulnerable to trawling.In 2004, some 1,134 scientists signed a statementurging the United Nations to declare a moratorium onbottom trawling on the high seas.How Have Human Activities AffectedMarine Fish Populations and Species? GoneFishing, Fish GoneAbout three-fourths of the world’s commerciallyvaluable marine fish species are overfished or fishednear their limits.Studies indicate that about three-fourths of the world’s200 commercially valuable marine fish species (40% inU.S. waters) are either overfished or fished to their estimatedsustainable yield. Overfishing is the greatestthreat to populations of fish that live in surface waters.Populations of bottom-dwelling fish are affected by acombination of overfishing and disruption of habitatby trawler fishing.In most cases, overfishing leads to commercial extinction.This is usually only a temporary depletion offish stocks, as long as depleted areas and fisheries areallowed to recover. But this is changing. Today fish arehunted throughout the world’s oceans by a global fleetof millions of fishing boats—some of them longer thana football field. These fleets, most supported by governmentsubsidies, use sonar, satellite global positioningsystems, and aircraft to find fish. Then they catchthem by deploying gigantic nets or lines containingmany thousands of hooks that can stretch as far as 80kilometers (50 miles). Modern industrial fishing cancause 80% depletion of a target fish species in only10–15 years.One result of the increasingly efficient global huntfor fish is that big fish in many populations of commerciallyvaluable species are becoming scarce. In 2003, fishery scientistsRansom Myers and Boris Worm looked at fishingdata for 13 commercial fisheries since 1952. Theirdata indicate that during the last 45 years the abundanceof large open-ocean fish such as swordfish, marlins,tunas, and sharks and bottom-dwelling groundfishsuch as cod plummeted by 90%! A 2004 study byJeffrey Hutchings and John Reynolds found that 230populations of marine fish have suffered an 83% dropin breeding population size from known historic levels.Many depleted species, like the bottom-dwellingNorth Atlantic cod, may never recover because toomuch of their habitat has been destroyed or degradedor there are too few survivors to find mates. For example,after stocks had dropped by 97–99% since the early1960s, Canada closed its cod fishery, putting thousandsout of work. After a decade there is no sign of recovery.The smaller fish are next. As the fishing industry hasdepleted its most valuable and larger species, it has begunworking its way down marine food webs to exploitsmaller and faster-growing varieties at lowertrophic levels (Figure 13-4). If this process continues, itwill begin to unravel food webs, disrupt marineecosystems, and hinder the recovery of fish feeding athigher trophic levels because the species they eat havealso been overfished.If this happens, the most abundant remainingspecies will be jellyfish, barnacles, and plankton. If wekeep vacuuming the seas, McDonald’s may beginserving barnacle burgers instead of fish sandwiches.Most fishing boats are after one or a small numberof commercially valuable species. However, their giganticnets and incredibly long lines of hooks alsocatch nontarget species, called bycatch. Almost onethirdof the world’s annual fish catch consists of suchspecies that are thrown overboard dead or dying. Inaddition to wasting potential sources of food, this candeplete the populations of bycatch species that playimportant ecological roles in oceanic food webs.Global freshwaterGlobal marine3.53.53.43.43.33.3Mean trophic level3.23.13.02.92.82.72.62.51950 1960 1970 1980 1990YearMean trophic level3.23.13.02.92.82.72.62.51950 1960 1970 1980 1990YearFigure 13-4 Meantrophic levels of theglobal marine (right)and freshwater (left)fish catch have declinedsince 1950.(Daniel Pauly)http://biology.brookscole.com/miller14255